Duplexer

ABSTRACT

Duplexers for high power applications are disclosed. In some embodiments, a duplexer includes 2N band pass filters, where N is an integer greater than 1. The 2N band pass filters each have an input and an output and are in an electrically parallel configuration. The duplexer includes a first adaptation circuit configured to couple a transmit signal received from a transmitter to each one of the 2N band pass filters. The duplexer includes a second adaptation circuit configured to couple outputs of the 2N band pass filters to an antenna, the second adaptation circuit providing an isolated path between the antenna and a receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No. 15/771,629, filed Apr. 27, 2018 entitled “DUPLEXER” which was a U.S. National Stage Application of International Application Number PCT/IB2015/058758, filed Nov. 12, 2015 entitled “DUPLEXER”, the entireties of both of which are incorporated herein by reference.

TECHNICAL FIELD

Wireless communications and in particular, duplexer designs for wireless transceivers.

BACKGROUND

A duplexer is a radio frequency (RF) component in an RF front end of a radio transceiver used in base stations and wireless devices in a wireless communication system. FIG. 1 shows a well-known duplexer design 2 used in many transceivers. The duplexer 2 has a transmit band pass filter (BPF) 4 and a receive BPF 6. The transmit BPF 4 is connected to a transmitter and is configured to pass signals having frequencies in the transmit frequency band F1, while rejecting signals at other frequencies, including rejecting signals at the receive frequency band F2. The receive BPF 6 is connected to a receiver and is configured to pass signals having frequencies in the receive frequency band F2, while rejecting signals at other frequencies, including rejecting signals at the transmit frequency band F1. Both BPFs 4 and 6 are connected to an antenna. Ideally, transmit signals in the transmit frequency band F1 from the transmitter are passed through the transmit BPF 4 to the antenna but are blocked from reaching the receiver by the receive BPF 6. Likewise, signals received by the antenna in the receive frequency band F2 are passed by the receive BPF 6 but blocked by the transmit BPF 4.

In reality, no BPF provides perfect isolation of out-of-band signal frequencies so that some of the transmit energy from the transmitter will leak through the BPF 6 into the receiver. Further, passive intermodulation (PIM) generated by the transmit BPF 4 may pass the receive BPF 6 and be received by the receiver. Note that since the transmit BPF 4 and the receive BPF 6 are directly connected at the antenna port, a stringent out of band attenuation requirement must be met in order to limit this leakage. Further, the power-handling capability of this type of duplexer design is mainly determined by the transmit BPF design.

Currently, only two types of small duplexers are commercially available: an acoustic type and a ceramic type. The acoustic type may be a surface acoustic wave (SAW), bulk acoustic wave (BAW) or film bulk acoustic resonator (FBAR). The ceramic type includes monoblock duplexers and ceramic coaxial duplexers. Whether the acoustic type or the ceramic type is used may depend on the power handling requirements of the transceiver and the maximum leakage tolerable at the receiver. The choice further depends on size, cost and weight constraints.

In general, for a radio design with transmit power averaging less than about 23 dBm, the acoustic type duplexers can meet entire performance requirements of the handset designs, but cannot fully meet the performance requirements of some base stations such as small cell base station designs. Some base stations require very high isolation between the transmit and receive ports, especially in the cases of high transmit power, which have not been achievable by acoustic type duplexer designs, and consequently, ceramic filters are typically used in these cases.

A disadvantage of ceramic filters is their size. A typical ceramic type duplexer may be of the dimensions of 52×14×6 millimeters (mm), whereas a typical acoustic type duplexer may be of the dimensions 2×1.6×0.6 mm. Hence, a typical ceramic type duplexer may be over 2000 times larger than an acoustic type duplexer. In addition to large size, ceramic type duplexers may be 100 times heavier and 10 times more costly than acoustic type duplexers. Further, acoustic type duplexers have Q factor that may be three times greater than the Q factor of ceramic type duplexers. A Q factor is an indication of energy stored by a resonator divided by energy dissipated per cycle.

Advantage to the ceramic type duplexers over other designs include much higher transmit power handling capability and lower PIM at the receive port for the same power handling. Therefore, designs for high power with low PIM requirements may be limited to ceramic duplexers.

FIG. 2 shows a duplexer design 8 that can be used for higher power applications. The duplexer 8 has two electrically parallel transmit BPFs 4 a and 4 b, referred to collectively as BPFs 4, a receive BPF 6, and two 90° hybrid couplers 10 a and 10 b referred to collectively herein as hybrid couplers 10. A 90° hybrid coupler is a four port device that is used either to equally split an input signal into two paths or to combine two signals while maintaining isolation between them.

For example, the 90° hybrid coupler 10 a splits the input from the transmitter at port A into two equal magnitude signals that are output at ports B and C. In this example, port D is terminated with a 50 ohms load. Each output of the 90° hybrid coupler 10 a is input to a different transmit BPF 4. Each BPF 4 has a substantially identical band pass response configured to pass signals at a transmit frequency band F1. Each transmit BPF 4 is output to one of the inputs of the 90° hybrid coupler 10 b via ports B and C.

The 90° hybrid coupler 10 b combines the inputs at ports B and C and outputs the combined signal at port A to an antenna. A signal received from the antenna is received at port A and split to two paths towards ports B and C, respectively. The split two signals are reflected at the ports B and C, and the reflected signals are combined at port D, which is coupled via the receive BPF 6 to a receiver. Since two band pass filters are used to filter the transmit signal, this type of duplexer might handle twice the transmit power of a duplexer having only one transmit BPF. Also, due to signal cancellation provided by the 90° hybrid couplers 10, this type of duplexer has much lower PIM at its receive port and much higher isolation between the transmit and receive ports of the duplexer as compared to the duplexer of FIG. 1.

However, if the two BPFs 4 were designed in SAW, BAW or FBAR filter technology, the balanced duplexer design of FIG. 2 can handle only twice the power of the duplexer design of FIG. 1 which is inadequate to meet the demands of a wide range of high power applications. Thus, in many cases, the bulky, heavy and expensive ceramic type duplexers are still used.

SUMMARY

The present embodiments advantageously provide duplexers that utilize filters such as acoustic wave, low temperature co-fired ceramic (LTCC) and other small form factor filters. According to one aspect, some embodiments include a duplexer for coupling between a transmitter and an antenna and for coupling between the antenna and a receiver. The duplexer includes 2N band pass filters, where N is an integer greater than 1. The 2N band pass filters each have an input and an output and are in an electrically parallel configuration. The duplexer includes a first adaptation circuit configured to couple a transmit signal received from a transmitter to each one of the 2N band pass filters. The duplexer includes a second adaptation circuit configured to couple outputs of the 2N band pass filters to an antenna, the second adaptation circuit providing a reflective path between an antenna and a receive port of the duplexer.

According to this aspect, in some embodiments, the first adaptation circuit includes a 90° hybrid coupler having at least one input and at least two outputs and configured to couple the transmit signal toward the 2N band pass filters. In some embodiments, each band pass filter has an impedance of Z₀ ohms and the 90° hybrid coupler has an impedance of Z₀/N ohms. In some embodiments, the duplexer further includes a quarter-wave transmission line between the transmitter and the hybrid coupler, the quarter-wave transmission line having a characteristic impedance of √{square root over ((Zin*Zo)/N)}, where Z_(in) is an input impedance of the transmitter. In some embodiments, the first adaptation circuit further includes a splitter circuit configured to direct an output of the 90° hybrid coupler to each of a plurality of the 2N band pass filters. In some embodiments, the splitter circuit includes a first splitter having N output ports and a second splitter having N output ports, each splitter receiving as an input an output of the 90° hybrid coupler. In some embodiments, the second adaptation circuit includes a 90° hybrid coupler configured to receive outputs of the 2N band pass filters, and to couple the received outputs to the antenna. In some embodiments, the second adaptation circuit includes a combiner circuit having a plurality of input ports, each input port receiving an output of one of a plurality of the 2N band pass filters. In some embodiments, the combiner circuit includes a first combiner having N input ports coupled to a first set of N output ports of the 2N band pass filters, and a second combiner having N input ports coupled to a second set of N output ports of the 2N band pass filters. In some embodiments, the first adaptation circuit includes a 90° hybrid coupler having at least two outputs and configured to receive the transmit signal, a splitter circuit configured to receive outputs of the 90° hybrid coupler, and an impedance transformer interposed between the 90° hybrid coupler and the splitter circuit to transform an impedance of the 90° hybrid coupler to an impedance of the splitter circuit. In some embodiments, the band pass filters are acoustic wave band pass filters, and in some embodiments the band pass filters are low temperature co-fired ceramic, LTCC, filters.

According to another aspect, some embodiments include a duplexer for coupling between a transmitter and an antenna and for coupling between the antenna and a receiver. The duplexer includes a plurality of band pass filters in an electrically parallel configuration and each band pass filter having an input and an output. The duplexer also includes a first adaptation circuit configured to direct a transmit signal to the plurality of band pass filters. The duplexer also includes a second adaptation circuit configured to direct outputs of the band pass filters to the antenna while providing a reflective path between the antenna and the receiver.

According to this aspect, in some embodiments, the first adaptation circuit includes a first 90° hybrid coupler configured to receive the transmit signal at an input port and output the transmit signal at a first output port and a second output port. The first adaptation circuit also includes a first splitter configured to receive a first output from the first output port and split the first output to a first plurality of paths, each path being coupled to a different one of a first set of the band pass filters. A second splitter is configured to receive a second output from the second output port, and split the second output to a second plurality of paths, each path being coupled to a different one of a second set of the band pass filters. In some embodiments, the duplexer further includes an impedance transformer positioned between the first 90° hybrid coupler and the first splitter to match an impedance of the first 90° hybrid coupler to an impedance of the first splitter. In some embodiments, the duplexer further includes an impedance transformer positioned between the first 90° hybrid coupler and the second splitter to match an impedance of the 90° hybrid coupler to an impedance of the second splitter.

In some embodiments, the second adaptation circuit includes a second 90° hybrid coupler having two input ports and two output ports, one output port configured to be coupled to the antenna and the other output port configured to be coupled to the receiver. A first combiner is configured to receive at each of a first plurality of inputs, an output of a band pass filter, and combine the received first plurality of inputs to produce an input to a first one of the two input ports of the second 90° hybrid coupler. A second combiner is configured to receive at each of a second plurality of inputs, an output of a band pass filter, and combine the received second plurality of inputs to produce an input to the second one of the two input ports of the second 90° hybrid coupler.

According to some embodiments, the duplexer further includes an impedance transformer positioned between the second 90° hybrid coupler and the first combiner to match an impedance of the second 90° hybrid coupler to an impedance of the first combiner. In some embodiments, the duplexer further includes an impedance transformer positioned between the first 90° hybrid coupler and the second combiner to match an impedance of the 90° hybrid coupler to an impedance of the second combiner. In some embodiments, the band pass filters include 2^(N) band pass filters, and wherein the first adaptation circuit includes N stages of 90° hybrid couplers. In some embodiments, the second adaptation circuit includes N stages of 90° hybrid couplers. In some embodiments, the plurality of band pass filters are acoustic wave band pass filters, and in some embodiments the plurality band pass filters are low temperature co-fired ceramic, LTCC, filters.

According to another aspect, a duplexer includes a first 90° hybrid coupler configured to be coupled to a transmitter and a load, the first 90° hybrid coupler having at least one output. The duplexer includes a second 90° hybrid coupler configured to be coupled to a receiver and to an antenna, the second 90° hybrid coupler having at least one input. A transmit band pass filtering circuit includes a plurality of band pass filters in an electrically parallel configuration, each band pass filter having an input and an output. The duplexer includes a first circuit interconnecting the first 90° hybrid coupler and the transmit band pass filtering circuit and a second circuit interconnecting the second 90° hybrid coupler and the transmit band pass filtering circuit.

According to this aspect, in some embodiments, the first circuit includes a splitter having an input coupled to an output of the first 90° hybrid coupler, the splitter having an output for each one of a plurality of the band pass filters. In some embodiments, the second circuit includes a combiner having an input for each one of the plurality of the band pass filters, the combiner having an output coupled to an input of the second 90° hybrid coupler. In some embodiments, the duplexer includes a first impedance matching circuit between the transmitter and the first 90° hybrid coupler and a second impedance matching circuit between the antenna and the second 90° hybrid coupler. In some embodiments, the plurality of band pass filters are acoustic wave band pass filters, and in some embodiments the plurality band pass filters are low temperature co-fired ceramic, LTCC, filters.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic of a known duplexer configuration;

FIG. 2 is a schematic of a known duplexer configuration for higher isolation between the transmit and receive ports;

FIG. 3 is a block diagram of a duplexer configuration having two coupler modules with four transmit band pass filters (BPF);

FIG. 4 is a schematic of an embodiment of a duplexer configuration having splitters and combiners;

FIG. 5 is a schematic of an embodiment of a 2N-filter duplexer having 1:N splitters and combiners;

FIG. 6 is a schematic of an embodiment of a two-stage coupler type balanced duplexer having 2² acoustic wave BPFs;

FIG. 7 is a schematic of an embodiment of a three-stage coupler type balanced duplexer having 2³ acoustic wave BPFs;

FIG. 8 is a schematic of an embodiment of an n-stage coupler type balanced duplexer having 2^(n) acoustic wave BPFs;

FIG. 9 is a schematic of an embodiment of a duplexer using quarter wave transmission lines for impedance matching;

FIG. 10 is a schematic of an embodiment of a duplexer using quarter wave transmission lines for impedance matching and having 2N acoustic wave BPFs;

FIG. 11 is a schematic of an embodiment of a duplexer using transformers for impedance matching; and

FIG. 12 is a schematic of an embodiment of a duplexer using transformers for impedance matching and having 2N acoustic wave BPFs.

DETAILED DESCRIPTION

Before describing in detail exemplary embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to using acoustic filters in duplexers for high power transceivers such as those used in wireless communication base station equipment. Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements.

In some embodiments, acoustic type band pass filters, such as SAW, BAW and FBAR band pass filters are used to design much smaller, lighter and cheaper duplexers than can be achieved with ceramic filters. In some embodiments, the acoustic type duplexers described herein can handle higher power with lower PIM and better Tx-Rx isolation than can be achieved with ceramic filters. Some embodiments described herein apply impedance matching technology for power splitting and combining. Of note, although this disclosure generally describes embodiments that use acoustic wave filters and duplexers based on acoustic wave filters, embodiments are not limited solely to the use of acoustic wave filters. It is contemplated that low temperature co-fired ceramic (LTCC) filters can be used as can other small form-factor filters.

FIG. 3 is a block diagram of a radio frequency (RF) duplexer constructed in accordance with principles described herein for use in a radio transceiver. An RF circuit board 11 has mounted thereon a first adaptation circuit 12 a and a second adaptation circuit 12 b, referred to collectively as adaptation circuits 12. In between the second adaptation circuit 12 b and the first adaptation circuit 12 a is a plurality of electrically parallel acoustic wave transmit BPFs 14. Although only four transmit BPFs 14 are shown, more transmit BPFs are included in some embodiments, as will be explained below. An acoustic wave receive BPF 16 couples an output D of the second adaptation circuit 12 b to a receiver of the transceiver. An input A of the first adaptation circuit 12 a is connected to the transmitter of the transceiver. The first adaptation circuit 12 a is configured to couple a transmit signal from a transmitter to each of the acoustic wave transmit BPFs 14. The output A of the second adaptation circuit 12 b is connected to an antenna and the output D of the second adaptation circuit 12 b is connected to a receiver. The second adaptation circuit 12 b is configured to couple the outputs of the acoustic wave transmit BPFs 14 to the antenna. The operation of the adaptation circuits is described in more detail below. The function of the first adaptation circuit 12 a is generally to split the transmit signal to multiple ports of the transmit BPFs and to match the impedance of the transmitter output to the impedance of the transmit BPFs. Similarly, the function of the second adaptation circuit is generally to combine the signals from the transmit BPFs to the output to the antenna and to match the impedance of the transmit BPFs to the impedance of the antenna.

FIG. 4 is a schematic diagram of one embodiment of a duplexer 15 having adaptation circuits 20 a and 20 b, referred to herein collectively as adaptation circuits 20, and four transmit BPFs 14. The adaptation circuits 20 a and 20 b are exemplary specific implementations of the coupler modules 12 a and 12 b. Adaptation circuit 20 a has two 2-port splitters 18 a and a 90° hybrid coupler 10 a. Adaptation circuit 20 b has two 2-port combiners 18 b and a 90° hybrid coupler 10 b. The splitters 18 a couple the output ports B and C of the 90° hybrid coupler 10 a to four transmit BPFs 14. The combiners 18 b combine the outputs of the four transmit BPFs 14 and direct the combined outputs to input ports B and C of the 90° hybrid coupler 10 b. The configuration of FIG. 4 has about four times the power handling capability as the configuration of FIG. 1 because of the use of four times the number of acoustic wave BPFs. Also, due to signal cancellation brought by the two 90° hybrid couplers 10 a and 10 b, the four equal power transmit signals output from the four transmit BPFs 14 will be cancelled at port D, resulting in improved isolation between the transmitter and receiver. Also, if there is any PIM generated by the four transmit BPFs 14 when passing the transmit signals, the PIM will cancel at the receive port D. Note that in this embodiment, the coupler 10 a has an input coupled to a 50 ohm load and the 90° hybrid couplers 10 a and 10 b each have 50 ohm impedance. Note also that the designation of a 50 ohm load will be understood by persons of skill in the art to be approximately 50 ohms, since it is unlikely that a resistive product will necessarily be exactly 50 ohms.

FIG. 5 is a schematic diagram of one embodiment of an acoustic type duplexer 21 having two adaptation circuits 24 a and 24 b, referred to herein collectively as adaptation circuits 24. The adaptation circuits 24 a and 24 b are exemplary specific implementations of the coupler modules 12 a and 12 b. The signal cancellation mechanism of the configuration of FIG. 5 is similar to the signal cancellation mechanism of the configuration of FIG. 4. The power handling capability of the configuration of FIG. 5 is about 2N times the power handling capability of the configuration of FIG. 1. In the embodiment of FIG. 5, adaptation circuit 24 a has two N-port splitters 22 a and adaptation circuit 24 b has two N-port combiners 22 b, where N is an integer greater than 1. Each of the two N-port splitters 22 a splits an output of the 90° hybrid coupler 10 a to N ports to feed N transmit BPFs 14, so that there are 2N transmit BPFs 14. Each of the two N-port combiners 22 b combine N outputs of N transmit BPFs 14 to produce a single output that is fed to an input port of the 90° hybrid coupler 10 b. Once again, the 90° hybrid coupler 10 a is connected to a 50 ohm input impedance and the hybrid couplers 10 a, 10 b, 22 a and 22 b have a 50 ohm impedance.

FIG. 6 is a schematic diagram of one embodiment of a two-stage coupler balanced duplexer 27 having four transmit BPFs 14 between adaptation circuits 26 a and 26 b. The adaptation circuits 26 a and 26 b are exemplary specific implementations of the coupler modules 12 a and 12 b. Adaptation circuit 26 a has a first stage 90° hybrid coupler 28 a which directs the transmitter input to two output ports B and C and two second stage 90° hybrid couplers 30 a. The outputs on output ports B and C are each input to a different one of the two second stage 90° hybrid couplers 30 a. The two outputs of each of the second stage 90° hybrid couplers 30 a are input to the four transmit BPFs 14. The outputs of the 4 BPFs 14 are input to adaptation circuit 26 b, which includes two stages of 90° hybrid couplers 28 b and 30 b. Specifically, the outputs of the four BPFs are input to one each of four inputs of the two second stage 90° hybrid couplers 30 b. The outputs of the two second stage 90° hybrid couplers 30 b are input to a final stage 90° hybrid coupler 28 b which directs the two inputs of the 90° hybrid coupler 28 b to the antenna. The power handling capability of the configuration of FIG. 6 is about four times the power handling capability of the configuration of FIG. 1 because the configuration of FIG. 6 uses four similar BPFs, and results in better isolation and lower PIM at the receive port D. An advantage to using two stages of 90° hybrid couplers rather than the splitters 18 a and combiners 18 b of FIG. 4 is that the adaptation circuits may be easier to build because 90° hybrid couplers are commercially available. Note that R0 is 50 ohm in the embodiment of FIG. 6.

FIG. 7 is a schematic diagram of one embodiment of a three-stage coupler type balanced duplexer 31 using 2³ transmit BPFs 14 between two adaptation circuits 34 a and 34 b. The adaptation circuits 34 a and 34 b are exemplary specific implementations of the coupler modules 12 a and 12 b. Adaptation circuit 34 a has three stages of 90° hybrid couplers 28 a, 30 a and 32 a. Adaptation circuit 34 b also has three stages of 90° hybrid couplers 28 b, 30 b and 32 b. This configuration provides about 2³=8 times the power handling capability of the configuration of FIG. 1. Note that R0 is 50 ohm in the embodiment of FIG. 7.

FIG. 8 is a schematic diagram of one embodiment of an n-stage coupler balanced duplexer 33, using 2′ transmit BPFs 14 and having n stages of 90° hybrid couplers 28 a, 30 a, and 32 a, in a first adaptation circuit 36 a and having n stages of 90° hybrid couplers 28 b, 30 b and 32 b, in a second adaptation circuit 36 b. The adaptation circuits 36 a and 36 b are exemplary specific implementations of the coupler modules 12 a and 12 b. The configuration of FIG. 8 is an extension of the configuration of FIG. 7 from three stages to n stages of 90° hybrid couplers. Because the configuration of FIG. 8 has about 2^(n) times the number of BPFs as the configuration of FIG. 1, the configuration of FIG. 8 has 2^(n) times the power handling as the configuration of FIG. 1. The configuration of FIG. 8 may also provide improved transmit-receive (Tx-Rx) isolation and PIM reduction at the receive port D of the hybrid coupler 28 b. Note that R0 is 50 ohm in the embodiment of FIG. 8.

FIG. 9 is a schematic diagram of one embodiment of a balanced duplexer design 39 having four transmit BPFs 14 between two adaptation circuits 38 a and 38 b. The adaptation circuits 38 a and 38 b are exemplary specific implementations of the coupler modules 12 a and 12 b. The configuration of FIG. 9 is especially adapted to handle BPFs with a characteristic impedance different than 50 ohms, generally designated as Z₀. FIG. 9 shows a load with an impedance of Z₀/2. It is understood, however, that the load impedance need not be exactly Z₀/2 and that reference to a load impedance of Z₀/2 is approximate. The adaptation circuit 38 a has a Z₀/2 ohm 90° hybrid coupler 40 a connected to a load with impedance of Z₀/2 ohms at port D and a quarter wave transmission line 42 a, where Z₀ is the characteristic impedance of the transmit BPFs 14. Splitters 41 a split the outputs B and C of the 90° hybrid coupler 40 a to the inputs of the four transmit BPFs 14. The adaptation circuit 38 b has combiners 41 b that combine the outputs of the transmit BPFs 14 and input the combined signals to a Z₀/2 ohm 90° hybrid coupler 40 b. The output ports A and D of the 90° hybrid coupler 40 b are input to quarter wave transmission lines 42 b and 42 c. The duplexer 39 has about four times the power handling capability as the configuration of FIG. 1, with improvement in the Tx-Rx ports isolation and PIM reduction at the receive (Rx) port.

In FIG. 9, if the antenna load impedance is Z₁₁, the receiver load impedance is Z₁₂, and the characteristic impedance of the hybrid coupler 40 b is Z₀/2, and the characteristic impedance of the transmit BPFs is Z₀, then the characteristic impedance of the quarter wave transmission line 42 b is

$\sqrt{\frac{Zo*Zl1}{2}}$

and the characteristic impedance of the quarter wave transmission line 42 c is

$\sqrt{\frac{Zo*Zl2}{2}}.$

If the input impedance of the transmitter is Z_(in), then the characteristic impedance of the quarter wave transmission line 42 a is

$\sqrt{\frac{Zin*Zo}{2}}.$

Note also that the characteristic impedance of the receiver BPF 16 is Z₁₂ so that the quarter wave transmission line 42 c matches the port impedances of the port D of the coupler 40 b and the receiver BPF. An advantage of the configuration of FIG. 9 is a simpler configuration as compared with others because the splitter 41 a and the combiner 41 b are comprised of just three transmission lines in a Y shape.

FIG. 10 is a schematic diagram of one embodiment of duplexer 41 having 2N transmit BPFs 14 between first and second adaptation circuits 47 a and 47 b. The adaptation circuits 47 a and 47 b are exemplary specific implementations of the coupler modules 12 a and 12 b. The embodiment of FIG. 10 is an extension of the embodiment of FIGS. 9 to 2N transmit BPFs. FIG. 10 shows a load with an impedance of Z₀/N. It is understood, however, that the load impedance need not be exactly Z₀/N and that reference to a load impedance of Z₀/N is approximate. The first adaptation circuit 47 a has a 90° hybrid coupler 44 a having a characteristic impedance Z₀/N, splitters 45 a 1 and 45 a 2, referred to herein collectively as splitters 45 a, and a quarter wave transmission line 46 a having an impedance of

$\sqrt{\frac{Zin*Zo}{N}}.$

The second adaptation circuit 47 b has a 90° hybrid coupler 44 b having a characteristic impedance Z₀/N, combiners 45 b 1 and 45 b 2, referred to herein collectively as combiners 45 b, a first quarter wave transmission line 46 b having a characteristic impedance of

$\sqrt{\frac{Zo*Zl1}{N}},$

and a second quarter wave transmission line 46 c having a characteristic impedance of

$\sqrt{\frac{Zo*Zl2}{N}},$

where Z₁₁ is the load impedance of the antenna and Z₁₂ is the load impedance of the receiver. Note also that the characteristic impedance of the receiver BPF 16 is Z₁₂. The configuration of FIG. 10 has about 2N times the power handling capability as the configuration of FIG. 1, because of the use of 2N times the number of transmit BPF filters. Note further that the splitters and combiners 22 of FIG. 5 are different from the splitters and combiners 45 of FIG. 10. The splitters and combiners 22 may be a circuit component such as a Wilkinson type power divider or combiner, whereas the splitters and combiners 45 may be just a set of transmission lines joined at a node.

FIG. 11 is a schematic diagram of one embodiment of a duplexer 47 having four transmit BPFs 14 between adaptation circuits 48 a and 48 b. The adaptation circuits 48 a and 48 b are exemplary specific implementations of the coupler modules 12 a and 12 b. Each BPF has a characteristic impedance of Z₀. The adaptation circuit 48 a has a 90° hybrid coupler 50 a having a characteristic impedance Z_(in) matching the input impedance of the transmitter. It is understood, however, that the load impedance need not be exactly Z_(in) and that reference to a load impedance of Z_(in) is approximate. Two 1:M transformers 52 a, each having a turns ratio of

${M = \sqrt{\frac{Zo}{2*Zin}}},$

couples the outputs of the 90° hybrid coupler 50 a to splitters 51 a and matches the impedance of the transmit BPFs 14 to the impedance of the 90° hybrid coupler 50 a. The outputs of the splitters 51 a are input to the 4 transmit BPFs 14. The outputs of the transmit BPFs 14 are input to the combiners 51 b which combine the outputs for input to the two M:1 transformers 52 b. The two M:1 transformers 52 b have a turns ratio

$\sqrt{\frac{Zo}{2*Zl}}$

where Z₁ is the impedance of the antenna and the characteristic impedance of the 90° hybrid coupler 50 b. The power handling capabilities of the duplexer of FIG. 11 is about four times the power handling capability of the duplexer of FIG. 1, because of the use of four transmit BPFs. An advantage of using transformers for impedance matching, rather than the quarter wave transmission lines of FIG. 10, is that transformers 52 may be low cost components and are usually commercially available. However, for some values of M, the transformers 52 may need to be specifically designed and may not be commercially available. Transformer design is known to those of skill in the art and beyond the scope of this disclosure.

FIG. 12 is a schematic diagram of one embodiment of a duplexer 57 having first and second adaptation circuits 54 a and 54 b with 2N transmit BPFs 14 in between. The adaptation circuits 54 a and 54 b are exemplary specific implementations of the coupler modules 12 a and 12 b. The configuration of FIG. 12 is an extension of the configuration of FIG. 11 to 2N transmit BPFs. Each BPF has a characteristic impedance of Z₀. The adaptation circuit 54 a has a 90° hybrid coupler 56 a having a characteristic impedance Z_(in) matching the input impedance of the transmitter. It is understood, however, that the load impedance need not be exactly Z_(in) and that reference to a load impedance of Z_(in) is approximate. Two 1:M transformers 58 a, each having a turns ratio of

${M = \sqrt{\frac{Zo}{N*Zin}}},$

couple the outputs of the 90° hybrid coupler 56 a to 1:M splitters 57 a 1 and 57 a 2, referred to herein collectively as splitters 57 a, and match the impedance of the transmit BPFs 14 to the impedance of the 90° hybrid coupler 56 a. The outputs of the splitters 57 a are input to the 2N transmit BPFs 14. The outputs of the transmit BPFs 14 are input to the M:1 combiners 57 b 1 and 57 b 2, referred to collectively herein as combiners 57 b which combine the outputs for input to the two M:1 transformers 58 b. The two M:1 transformers 58 b have a turns ratio

$\sqrt{\frac{Zo}{N*Zl}}$

where Z₁ is the impedance of the antenna and the impedance of the 90° hybrid coupler 56 b, and also the impedance of the receive BPF 16 or the receiver load impedance. The power handling capabilities of the duplexer of FIG. 12 is 2N times the power handling capability of the duplexer of FIG. 1, and improved Tx-Rx ports isolation and PIM reductions at the receive port may be achieved.

Note that the embodiments of FIGS. 3-12 generally all have greater power handling capabilities than the configurations of FIGS. 1 and 2, and generally achieve higher Tx-Rx ports isolation and PIM reduction at the receive port. Note also that, because the BPFs are acoustic wave filters, low cost, small size and light weight duplexers can be achieved. Different types of 90° hybrid couplers may be used including hybrid couplers having different characteristic impedances. Further, the embodiments of FIGS. 9-12 may be used to match different impedances of the receiver, transmitter, and transmit BPFs. Impedance matching is generally desirable to reduce standing wave ratios and to increase power transfer across the duplexer.

It will be appreciated by persons skilled in the art that embodiments are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims. 

What is claimed is:
 1. A duplexer, comprising: 2N band pass filters, N being an integer greater than 1, the 2N band pass filters each having an input and an output and being in an electrically parallel configuration; a first adaptation circuit configured to couple a transmit signal received from a transmitter to each one of the 2N band pass filters; and a second adaptation circuit configured to couple outputs of the 2N band pass filters to an antenna, the second adaptation circuit providing a reflective path between the antenna and a receiver.
 2. The duplexer of claim 1, wherein the first adaptation circuit includes a 90° hybrid coupler having at least one input and at least two outputs and configured to couple the transmit signal toward the 2N band pass filters.
 3. The duplexer of claim 2, wherein each band pass filter has a characteristic impedance of Z₀ ohms and the 90° hybrid coupler has an impedance of Z₀/N ohms.
 4. The duplexer of claim 3, further comprising a quarter-wave transmission line between the transmitter and the 90° hybrid coupler, the quarter-wave transmission line having a characteristic impedance of √{square root over ((Zin*Zo)/N)}, where Z_(in) is an input impedance of the transmitter.
 5. The duplexer of claim 2, wherein the first adaptation circuit further includes a splitter circuit configured to direct an output of the 90° hybrid coupler to each of a plurality of the 2N band pass filters.
 6. The duplexer of claim 5, wherein the splitter circuit includes a first splitter having N output ports and a second splitter having N output ports, each splitter receiving as an input an output of the 90° hybrid coupler.
 7. The duplexer of claim 1, wherein the second adaptation circuit includes a 90° hybrid coupler configured to receive outputs of the 2N band pass filters, and to couple the received outputs to the antenna.
 8. The duplexer of claim 7, wherein the second adaptation circuit includes a combiner circuit having a plurality of input ports, each input port receiving an output of one of a plurality of the 2N band pass filters.
 9. The duplexer of claim 8, wherein the first combiner circuit includes a first combiner having N input ports coupled to a first set of N output ports of the 2N band pass filters.
 10. The duplexer of claim 9, wherein the first combiner circuit includes a second combiner having N input ports coupled to a second set of N output ports of the 2N band pass filters.
 11. The duplexer of claim 1, wherein the first adaptation circuit includes a 90° hybrid coupler having at least two outputs and configured to receive the transmit signal, a splitter circuit configured to receive outputs of the 90° hybrid coupler, and an impedance transformer interposed between the 90° hybrid coupler and the splitter circuit to transform an impedance of the 90° hybrid coupler to an impedance of the splitter circuit.
 12. The duplexer of claim 1, wherein the band pass filters are acoustic wave band pass filters.
 13. The duplexer of claim 1, wherein the band pass filters are low temperature co-fired ceramic, LTCC, filters.
 14. A duplexer for coupling between a transmitter and an antenna and for coupling between the antenna and a receiver, the duplexer comprising: a plurality of band pass filters in an electrically parallel configuration and each band pass filter having an input and an output; a first adaptation circuit configured to direct a transmit signal to the plurality of band pass filters; and a second adaptation circuit configured to direct outputs of the band pass filters to the antenna while providing a reflective path between the antenna and the receiver.
 15. The duplexer of claim 14, wherein the first adaptation circuit includes: a first 90° hybrid coupler configured to receive the transmit signal at an input port and output the transmit signal at a first output port and a second output port; and a first splitter configured to: receive a first output from the first output port; and split the first output to a first plurality of paths, each path being coupled to a different one of a first set of the band pass filters; and a second splitter configured to: receive a second output from the second output port; and split the second output to a second plurality of paths, each path being coupled to a different one of a second set of the band pass filters.
 16. The duplexer of claim 15, further comprising an impedance transformer positioned between the first 90° hybrid coupler and the first splitter to match an impedance of the first 90° hybrid coupler to an impedance of the first splitter.
 17. The duplexer of claim 15, wherein the second adaptation circuit includes: a second 90° hybrid coupler having two input ports and two output ports, one output port configured to be coupled to the antenna and the other output port configured to be coupled to the receiver; a first combiner configured to: receive at each of a first plurality of inputs, an output of a band pass filter; and combine the received first plurality of inputs to produce an input to a first one of the two input ports of the second 90° hybrid coupler; a second combiner configured to: receive at each of a second plurality of inputs, an output of a band pass filter; and combine the received second plurality of inputs to produce an input to the second one of the two input ports of the second 90° hybrid coupler.
 18. The duplexer of claim 17, further comprising an impedance transformer positioned between the second 90° hybrid coupler and the first combiner to match an impedance of the second 90° hybrid coupler to an impedance of the first combiner.
 19. The duplexer of claim 14, wherein the band pass filters include 2^(N) band pass filters, and wherein the first adaptation circuit includes N stages of 90° hybrid couplers.
 20. A duplexer, comprising: a first 90° hybrid coupler configured to be coupled to a transmitter and a load, the first 90° hybrid coupler having at least one output; a second 90° hybrid coupler configured to be coupled to a receiver and to an antenna, the second 90° hybrid coupler having at least one input; a transmit band pass filtering circuit comprising a plurality of band pass filters in an electrically parallel configuration, each band pass filter having an input and an output; a first circuit interconnecting the first 90° hybrid coupler and the transmit band pass filtering circuit; and a second circuit interconnecting the second 90° hybrid coupler and the transmit band pass filtering circuit. 